The present invention concerns a method for measuring the time delay between a first periodic pulse signal and a second periodic pulse signal having the same frequency as the first pulse signal.
The invention also concerns a method for measuring the separation between two objects using a sensor disposed on one of the objects, wherein
the sensor triggers a first periodic pulse signal and transmits it towards the other object, PA1 the first pulse signal, reflected from the other object and delayed by a propagation time dependent on the separation between the two objects, is received by the sensor, PA1 the sensor triggers a second periodic pulse signal delayed by a variable time delay, PA1 the variable time delay of the second pulse signal is varied, and PA1 the variable time delay is determined which is equal to the propagation time of the received reflected first pulse signal. PA1 first means for triggering and for transmitting a first periodic pulse signal towards the other object, PA1 second means for receiving the first pulse signal reflected from the other object which is delayed by a propagation time in dependence on the distance between the two objects, PA1 third means for triggering a second periodic pulse signal delayed by a variable time delay, PA1 fourth means for varying the variable time delay of the second pulse signal, and PA1 fifth means for determining the adjusted variable time delay which is equal to the propagation time of the first received reflected pulse signal. PA1 production of a periodic diagnostic signal having half the frequency of the first pulse signal, whose sampling ratio is a measure for the time delay between the first pulse signal and the second pulse signal, PA1 determination of the sampling ratio of the diagnostic signal, and PA1 determination of the time delay between the first pulse signal and a second pulse signal using the sampling ratio of the diagnostic signal. PA1 a control signal is produced having half the frequency of the first pulse signal, PA1 the inverted control signal is introduced to a first control input of a multiplexer, PA1 the first pulse signal is introduced to a first signal input of the multiplexer, PA1 the control signal is introduced to a second control input of the multiplexer, PA1 the second pulse signal is introduced to a second signal input of the multiplexer, PA1 the output of the multiplexer is introduced to a clock input of a first D flip-flop, PA1 the inverted control signal is introduced to the D-input of the first D flip-flop, and PA1 the diagnostic signal is present on an output of the first D flip-flop. PA1 6.sup.th means for producing a periodic diagnostic signal having half the frequency of the first pulse signal, whose sampling ratio is a measure for the time delay between the first pulse signal and the second pulse signal, PA1 7.sup.th means for determining the sampling ratio of the diagnostic signal, and PA1 8.sup.th means for determining the time delay between the first pulse signal and the second pulse signal from the sampling ratio of the diagnostic signal. PA1 means for producing a control signal having half the frequency of the first pulse signal, PA1 a multiplexer having a first input to which the inverted control signal is applied, and having the first pulse signal present on its second input, wherein its third input has the control signal, and its fourth input the second pulse signal, and PA1 a first D flip-flop having the output of the multiplexer on its clock input and the inverted control signal on its D input, wherein its output is the diagnostic signal. PA1 In accordance with another preferred embodiment, the multiplexer comprises three NAND gates, wherein the inverted control signal is applied to a first input of the first NAND gate and the first pulse signal is applied to a second input of the first NAND gate, wherein the control signal is applied to a first input of the second NAND gate and the second pulse signal is applied to the second input of the second NAND gate, wherein the output of the first NAND gate is applied to a first input of the first NAND gate and the output of the second NAND gate is applied to a second input of a third NAND gate, and wherein the output of the third NAND gate is applied to the clock input of the first D flip-flop. A multiplexer made in this fashion from three NAND gates is particularly inexpensive. In this manner, the device in accordance with the invention does not require particularly fast and therefore very expensive components. Despite the relatively slow switching times and necessary stable signal holding times for the standard components utilized in the device in accordance with the invention (D flip-flops and NAND gates), a proper circuiting of these components leads to the reliable and highly precise determination of small delays between two pulse signals. In this fashion, even delays which are less than the above mentioned internal switching times and required holding times can be easily determined.
The invention also concerns a device for measuring the time delay between a first periodic pulse signal and a second periodic pulse signal having the same frequency as the first pulse signal.
Finally, the present invention concerns a sensor for measuring the separation between two objects, wherein the sensor is disposed on one of the objects and has:
Measurement of the time delay between two periodic pulse signals having the same frequency is done for a plurality of different applications. A preferred application is the measurement of the distance between two objects. The objects can e.g. be motor vehicles. Conventionally, the separation between two motor vehicles, driving one behind the other, is kept constant through regulation of the speed of the trailing motor vehicle. Towards this end, the trailing motor vehicle has a sensor of the above mentioned kind in its forward region for measuring the distance from the leading motor vehicle.
In order to measure the distance between two objects using a sensor disposed on one of the objects, sensors are normally used which determine the propagation time of a first periodic pulse between its transmission by the sensor, its reflection on the other object, and up to receipt by the sensor. The propagation time of the first pulse signal is given by twice the separation between the two objects divided by the propagation speed of the first pulse signal. In this manner, the propagation time of the received first pulse signal can be utilized to determine the separation between the two objects. Ultrasonic sensors and pulse radar sensors operate on this principle. Ultrasonic sensors (radiating pulse signals in the form of sound waves) have a propagation speed for the pulse signals approximately corresponding to the speed of sound (in air approximately 330 m/s) and pulse radar sensors (electromagnetic waves are radiated as pulse signals) have the speed of light (in air approximately 300.multidot.10.sup.6 m/s). In particular for pulse radar sensors, the propagation times which are to be determined, assuming a maximum measuring region of a few meters, lie in the nanosecond region. Assuming a measurement region of e.g. 10 m, the propagation time of the signal is approximately 66.multidot.10.sup.-9 s=66 ns. Since this pulse signal propagation time is extremely short, such pulse propagation times cannot be directly measured using e.g. a clock set with a START when the signal is transmitted and a STOP upon receipt of the reflected signal. Rather, indirect comparative methods are utilized to determine the pulse propagation times.
These comparative methods operate e.g. according to the following principle. A sensor pulse repetition generator triggers the first periodic pulse signal. The first pulse signal requires a certain propagation time between its transmission by the sensor, its reflection on the other object and up to reception by the sensor. In order to determine this propagation time, the sensor or the pulse repetition generator triggers a second periodic pulse signal having the same frequency as the first pulse signal but delayed by a variable time delay relative to the first pulse signal using a variable dead time component within the sensor. The control of the variable dead time member is effected using a relatively slowly varying sweeping voltage.
Both the received first pulse signal as well as the delayed second pulse signal are introduced to a mixer which delivers a maximum output signal when the propagation time of the received first pulse signal is precisely equal to that of the delay of the second pulse signal, as adjusted via the variable dead time component.
It would theoretically be sufficient to solely control the sweep of the internal delay through a time range corresponding to the measuring range of the sensor. In the event that the sensor e.g. has a measurement range of 10 m to 1000 m, the delay in a variable dead time component could be varied in the time range from 66 ns up to 6.6 .mu.s by continuously changing the sweep voltage.
Such a pure control procedure has however the disadvantage that drifts in the dead time properties of the dead time components due to the operating temperature of the sensor and aging processes in the sensor as well as to differences in manufactured characteristics of various components of the sensor can not be taken into consideration. Since the dead time component is normally analog, a drift in the dead time properties is hardly avoidable. For this reason, one cannot rely on a pure control procedure only and a regulation process must be used. The measurement procedure is then as follows. The sweep voltage is continuously and repetitively swept (e.g. a saw tooth) through a certain range so that the internal delay of the dead time component is varied over time e.g. from 0 ns to 40 ns (corresponding to a measurement region of approximately 0 m to 6 m). If, during this process, the reflected first pulse is incident on the receiver means having an external propagation time which corresponds to the instantaneously adjusted internal delay, this results in a detect able increase in the output signal on the mixer. one then checks which internal delay had been adjusted at this point in time of maximum mixer output. This delay can be determined using the sweep voltage itself to avoid problems resulted from unknown measured quantities (due to aging effects and component variations) when converting the sweep voltage into the internal delay. Another possibility is to measure the pulse sides of the first and the second pulse signals and the rely determine the adjusted delay. Therefore, a regulation procedure in the classical sense is not carried out, rather a measurement of instantaneous adjusted values of the delay. A certain degree of regulation is first effected in a second step when an evaluation unit (e.g. a micro-controller) "learns", as a result of the measurements, which sweeping voltage corresponds to which delay and takes this into consideration in the subsequent measurements. Such a learning process, compensating for aging and temperature related drifts in the internal delay unit, requires introduction of a time measurement component. This patent application concerns such a time measurement component.
In order to effect such a regulation, the time delay between the transmitted first pulse signal and the second internally delayed pulse signal must be determined. In accordance with prior art, the delay is simply assumed to be the setting of the dead time component. Thereby, as already mentioned above, drifts in the dead time properties of the dead time component as a function of sensor operation temperature and as a function of aging of the sensor as well as due to deviations in the manufactured properties of the differing components of the dead time component cannot be taken into account. For this reason, the time delays determined in accordance with prior art can deviate strongly from the actual delays which, under certain circumstances, leads to a highly inaccurate result for measurement of the separation between two objects.